1. Field of the Invention
The present invention relates to a method for determining the tendency of petroleum fractions e.g., crudes and residues, to foul heated metal surfaces such as heat exchangers.
2. Related Art
Different crude oils have different precipitating and fouling characteristics with regard to heated oil refinery surfaces. The problem of predicting the offending substances in a particular crude il which foul heat exchanger equipment in oil refineries and petrochemical plants has been virtually unresolved. Fouling of hydrocarbon streams, consisting of carbonaceous deposits on heat exchanger surfaces, leads to a blockage of flow and a decrease in heat transfer. Both resulting conditions severely reduce efficiency in the processing of the crude oil. If it can be predicted which fractions of the crude oils are troublesome, measures can be taken in advance to prevent this fouling by either removing the offending substances causing the deleterious deposits, or by adding antifouling additives to the flow stream to reduce deposit formation. Therefore, it would be most desirable to be able to predict these fouling substances.
There are a number of methods available for determining the rates of fouling of hydrocarbon streams. Conceptually, they are all similar in that they attempt to measure the change in heat transferred from a heated surface to a test fluid.
One approach is to use a test unit which is configured to allow measurement of the fluid temperature at the exit of the heat exchanger while the metal temperature of the heated tube is controlled, which is generally referred to as the Thermal Fouling Tester (TFT). This configuration provides for close simulation of refinery and petrochemical plant heat exchanger operations and provides for measurement of the significant effect of fouling which is indicated by the reduction of heat transfer. The test unit provides for a thermal fouling evaluation of the crude oil in an accelerated test which is designed to reproduce the fouling problem experienced in a refinery over several months. Acceleration is provided by carrying out test operating temperatures higher than those in a particular refinery unit, so that the prospective level of fouling can be produced in a reasonable period of time (usually 3-4 hours). Heat transfer data is obtained by holding the heater tube at a constant temperature, while measuring the change in the liquid outlet temperature. As fouling progresses, i.e., a carbonaceous deposit build up on the heater tube surface, a decrease in the fluid outlet temperature results when using a constant outlet liquid temperature operation. The change in liquid outlet temperature with time provides the basic heat data required for comparative evaluation of untreated material and additive-treated material. The rate of change in outlet liquid temperature versus time shows relative fouling tendencies. The duration of this test is usually three hours or longer. In practice in the laboratory, the turnaround for a single sample in this equipment is about one man day to obtain results. And one test unit will generally produce about 200 tests in a year. However, refinery feeds and streams change constantly and multiple tests, e.g., 10 to 50 or 60 per day may be required for full evaluation and control of the refinery operations.
Tests have shown that in fouling crude oils, the saturates fractions are generally in excess of 75 wt. % of the total crude and its aromatic fractions tend to be less than 25 wt. % of such fouling crude oils, which provided at least an empirical measure of determining the tendency of various petroleum fractions to foul.
Crude oils, being fluids can be fractionated into three separate and specific components: a hydrocarbon saturate fraction; a neutral hydrocarbon aromatic fraction; and a third fraction comprising aromatic components containing sulfur, oxygen and nitrogen (polar aromatic fraction).
Liquid chromatography is used to separate and quantify the aforementioned fractions in crude oils or other oils and petroleum fractions that are characterized by a tendency to foul refinery equipment. When these fractions are compared with the fractions similarly obtained from nonfouling crude oils, the substances causing fouling were identified. The fouling characteristics of crude oils or other petroleum fluids is measured by determining quantitively the composition of the petroleum fluid by techniques well known in the art such as Liquid Chromatography. The petroleum fluid is separated quantitively into three previously identified specific fractions, i.e.; a hydrocarbon saturate fraction, a neutral hydrocarbon aromatic fraction and a third fraction comprising the aromatic fraction containing sulfur, oxygen and nitrogen (polar aromatic fraction). The amount of neutral aromatics and polar aromatics present in the hydrocarbon fluid gives valuable information to predict the fouling tendency of the fluid to be tested.
It is an advantage of the present invention that a rapidly performable and quantitative method for determining the fouling tendency of a petroleum fraction is provided. Hence there is provided a method of predicting the fouling which results from the refinery treatment of any given petroleum stream. These and other advantages and features of the present invention will become clear from the following description.